Variable blurring of edges can be used to bring more attention to certain subjects/shapes by making them harder edged than others.
Variable blur on the edges of a single object/character/shape can also be used to indicate where a form is rounded as it recedes in space from the viewer (creating a softened edge) and where a form is more angular at its edge, turning sharply away from the viewer (creating a harder edge.)
Also, where the side of a subject/shape that is most brightly lit is seen against a dark background, a hard edge can emphasize this contrast. Likewise, a hard edge can emphasize the contrast between the shadow side of an object and a lighter background.
Variable blurring can also create a photographic look when objects on a certain focal plane have harder edges and objects in front of and behind this plane have progressively softer edges as they move away from the focal plane.
Featured: Market Place by John Singer Sargent.